Various electronic control units (ECUs) have been installed in vehicles with progress of introducing electronic techniques into the vehicles. An electronic control unit is provided to control a corresponding in-vehicle device. An in-vehicle device for receiving radio signal composes a keyless entry system or a tire pressure monitoring system, for example. The in-vehicle device for receiving radio signal is known as an in-vehicle device which is controlled by such an electronic control unit.
The in-vehicle device for receiving radio signal receives and demodulates a radio signal transmitted from a transmitter, and transmits the demodulated signal to an electronic control unit in order to control the in-vehicle device. For the purpose of performing such processing, an IC for receiving radio signal is installed in the in-vehicle device.
The electronic control unit and the in-vehicle device for receiving radio signal are connected via a wire harness. Control signals outputted from the electronic control unit are transmitted to the IC for receiving radio signal via the wire harness. Cables of the wire harness are assigned to the control signals respectively. Significances of the control signals are transmitted as DC voltage values via the respective cables, which enables static signal transmission of the significances.
In recent years, such an IC for receiving radio signal has progressed to have multi-functions so that the number of control signals tends to increase. The increase of the number of the control signals causes a problem that the number of cables of the wire harness increases according to a conventional static signal transmission.
In order to suppress increase of the number of the cables of the wire harness, a serial data transmission method may be employed. In the serial data transmission method, a plurality of control signals are converted into serial data and transmitted. However, the inside of a vehicle is an environment with a large amount of noise generally so that the employment of the serial data transmission may cause bit error of data due to the noise.
In order to suppress increase of the number of cables, another method may be employed. The method is that part of functions of an electronic control unit is integrated into a sub-microcomputer and that the sub-microcomputer is installed in an in-vehicle device for receiving radio signal. The sub-microcomputer controls an IC for receiving radio signal in the in-vehicle device.
In this method, the functions of the electronic control unit are divided so that it is possible to reduce the number of control signals to be outputted from the electronic control unit. However, such a sub-microcomputer can be a complicated circuit and the complicated circuit is installed in an in-vehicle device for receiving radio signal, which causes a problem of increase of failure rate of the in-vehicle device.